Success tastes good. Very good.

I completed another biofilm test on e.coli this week, and this time, the bacteria didn’t get totally wiped out by test tube bleach. Instead, basically what we found out was that in extremely low concentrations, neither dish detergent had too huge of an impact on the biofilms. We noted a 0.5 reduction of bacteria (about 50% of the bacteria were gone) in the presence of Dawn and a very small reduction with Cascade. This means that the two dish detergents did not really kill any of the biofilms of e.coli that you would potentially find in your gut. 

e.coli plates

Detergent plates: Left-Dawn, Right-Cascade

When I diluted and plated the detergents, I found that there was significant bacterial growth, which means that both detergents swept away biofilms that were still able to grow. Most dishwashing detergents can be classified as surfactants, meaning that they lower the surface tension between two liquids or a liquid and a solid. Surfactants are part of the reason why it is easier to scrub away food residue from plates when using dish detergent as opposed to just water. So in this case, the dishwashing detergents acted as surfactants for the e.coli biofilms, sweeping them away from the colonies left on the well plate.

 

So what does this mean for you?

Well, basically, at least in terms of e.coli, this means that small concentrations of dishwashing liquid are not harmful to the bacteria. So if you’re in love with using that bottle of Dawn with the adorable little duckling on it to wash your dishes, you’re probably ok, because it’s not going to screw up your intestinal e.coli super badly. However, since both detergents seemed to sweep away bacterial biofilms, accidentally consuming small quantities of dish soap could move bacterial colonies from one part of your intestine to another. And since science hasn’t discovered enough about gut flora yet, we are unsure of whether this movement would be harmful. Just don’t chug a bottle of Dawn, please. That’s a really bad idea.

За здоровье! To your health!

Mackenzie

Just to let you guys know, I will probably be posting every Friday from now on, for the sake of having complete results.
 I decided to use a new concentration of dish soap for the biofilm tests that I did this week and that I'll carry out over the next couple of weeks. Instead of 10 mg/mL dish soap, the concentration will be 0.1 mg/mL. Here's why:
 Imagine you're washing a plate, by hand, with liquid detergent. You put one drop of soap on the plate and kind of swirl it around to get good coverage. Then you scrub, rinse, and dry. That drop of soap you put on the plate probably weighed about 0.1 g, or 100 mg. Now mix that with 10 mL of water, the water that you sipped at dinner or the water with which you washed the plate--that's a 10 mg/mL solution. But that's still an awfully large concentration of soap. If you ate off a plate coated in 10 mg/mL Dawn, you would probably taste more soap than food. So imagine that you rinse that plate REALLY well, how it should be rinsed. At this point, you can assume you've rinsed off the majority of the detergent, and thus 0.1 mg/mL of soap is probably a pretty good estimate of the maximum amount of dish soap you would eat off the plate, a hundred times less soap than you had before.

Now, this is the moment of truth.Science requires repetition.

When I walked into the lab Thursday morning and opened the incubator, I found that many of my plates had absolutely no bacteria growing on them at all. When I did my control enumerations, two of my three trial plates were completely empty. This didn't make sense, especially after following all the procedures and using a much smaller concentration of dish soap. But since there was no dish soap in my control row, the only logical explanation I could assign is that there was bleach in my test tubes that killed the bacteria, even though the tubes had been autoclaved. This being said, I will have to re-do my e.coli enumerations next week. But no biggie- I still have another six weeks or so, and three of those weeks have been set aside for this kind of thing.

My poor little plates...

I did the same biofilm challenge procedure with this new concentration, except I did one thing a little different. Instead of shaking off the detergent before sonicating the plate, I collected the detergents in test tubes and plated them using serial dilutions. This was done to determine whether the dish soaps penetrated sand killed bacteria inside the biofilm or simply swept them away. The detergent plates surprisingly grew more than my control plate, and still grew quiet a bit, which leads me to believe it's possible that the dish soaps are sweeping the biofilms away. However, this is unsure until I can get solid, unbleached data.

Detergent plates- top:Cascade, bottom: Dawn

But for now, I'm off to Oregon for spring break, so,
увидмся через две недели! (See you in two weeks!)

Mackenzie

And... we have results!

Last week's biofilm tests had invalid (and unreadable) data due to the agar plates evaporating, but I re-did the 24-hour biofilm tests for e.coli and a solution of 10 mg/mL Dawn and now we have numbers! Woot! The only big difference between last week's biofilm tests and those for this week was that rather than letting the plates sit in the hood for the weekend, we just put them in the  incubator overnight to keep the pesky agar form going awry. Below are some pictures of the plates after counting colonies:

My beautiful plates!

E.coli Dawn challenge plates

E.coli control plates

Since making you sift through six nasty, ugly-looking data tables would be rather cruel, here is a brief summary of the results:
The Dawn had a HUGE effect on the quantity of e.coli biofilms.

For example, a plate count we used for the 1st control group was 3.9E7 CFU/mL.
The corresponding Dawn challenge enumeration was 4.8E4 CFU/mL.
 The CFUs/mL for the 2nd group control and Dawn, respectively, were 1.8E7 and 4.62E5.
And for the 3rd group: 5.9E6 and 2.26E5.

This means that the Dawn detergent is greatly decreasing the quantity of plated colonies. However, we do not know whether this concentration of detergent is killing the bacteria in the biofilms or if it is just sweeping them away. I will probably not have time to perform that kind of test, but based on the MICs, we are fairly sure that the detergent is sweeping the bacteria away. It's interesting to see that the Dawn is actually having an effect on the e.coli biofilms, as this was fairly unexpected. But don't make any assumptions yet! I will probably decrease the concentration of detergent to a more realistic figure, like 5 or 3mg/mL in coming weeks.There is also a pretty good chance I will do this exact thing again with this different concentration.

I haven't really had many obstacles except for getting used to the procedures, and everything seems pretty smooth.This kind of procedure is probably what you guys will be seeing for the remainder of my project. But don't worry! I will test different types of bacteria with different types of detergent, so hopefully, you'll have an update on that every week.

до четверга (until Thursday),

Mackenzie